By powder seeds are meant seeds of very small dimensions. Reduction in size, if carried far enough, greatly facilitates dispersal by wind. This is because the resistance offered by the air is relatively greater for a smaller body than for a larger one, so that rate of fall decreases as the size of the falling body diminishes--we all know how even a heavy material, if reduced to powder, will fall more slowly than when forming a single mass. Most of the spores of the “Flowerless Plants”--Ferns, Mosses, Fungi, etc.--are exceedingly minute, and have as a result a very slow rate of fall, and a consequent power of long-distance dispersal by wind. For instance, the microscopic spore of the puff-ball _Lycoperdon_ falls so slowly that, if we take again Guppy’s Azores example, it could traverse the 800 miles in a 50 miles an hour gale if it commenced its flight only 86 feet above the ground. Such spores are, in fact, so buoyant that they form a normal constituent of the air--as we know, for instance, by the rapidity with which they will discover and germinate upon a piece of cheese, forming bluemould--and with little doubt they are capable of reaching under favourable circumstances the most distant of oceanic islands. But in the Flowering Plants with which we are mainly concerned reduction in size is not carried far enough to confer any great amount of buoyancy. The minute seeds of the Poppies (_Papaver_), for instance, fall about 10 feet in a second. Applying again Guppy’s Azorean case, we find that though these would cover the distance in sixteen hours, they would fall in that time about 100 miles, unless raised during the journey to that extent by the excess of upward eddies as compared with downward ones--a quite impracticable proposition. In the Orchids alone do we find among the powder-seeded Flowering Plants a really effective buoyancy; this is due to the fact that great reduction in size is accompanied by very loosely disposed tissue enclosing the seed in a kind of net, and by the resistance to the air thus offered, greatly reducing the rate of fall. The seed of the Marsh Helleborine (_Epipactis longifolia_) falls only about 1/15 as fast as that of the Poppies, and would thus, under the same conditions, be carried fifteen times as far.